Known in the art are electrochemical cells comprising an anode made of an alkali metal, preferably lithium possessing a high negative potential, an inert porous cathode with electron-type conductivity, a cathode depolarizer dissolved in a non-aqueous electrolyte and reduced on an inert cathode, or a cathodic polarizer in a solid-phase state.
Electrolytes should meet certain requirements, namely: high ionic conductivity, compatibility with electrode materials, capability of dissolving products of discharges. As the vehicle for electrolytes use is made of organic solvents such as propylenecarbonate, acetonitrile, ethylenecarbonate, dimethylsulphite, dimethoxyethane or mixtures thereof.
The use of mixed solvents makes it possible to enhance ionic conductivity of the electrolyte, while retaining low viscosity solution. Along with organic solvents use can be also made of inorganic solvents such as oxyhalides of sulphur, phosphorus, selenium, thiohalides of phosphorus. They are resistant to the effect of, for example, lithium and can be used in combination therewith as an active cathode depolarizer.
An ionic component of the electrolyte is represented by salts highly soluble in organic solvents. These are exemplified by lithium salts such as perchlorate, tetrafluoroborate, and hexafluorophosphate. The basic requirement imposed on salts is their inactivity relative to the cathode depolarizer and the anode metal.
As regards the state of the cathodic depolarizer, the above-mentioned electrochemical cells can be divided into two groups;
(a) electrochemical cells with a solid-phase cathodic active substance (oxides of metals, chromates, halides of metals, and sulphides), wherein the reduction of a cathodic reagent occurs in most cases according to the solid-phase mechanism; PA1 (b) electrochemical cells with a cathodic active substance such as sulphur dioxide, thionyl chloride and the like dissolved in the electrolyte. The cathodic reduction of the active substance dissolved in the electrolyte proceeds on the surface of an inert electrode possessing such electron conductivity which enables unhindered transfer of electrons during an electrochemical reaction.
In a chemical current source of the system Li-SO.sub.2 during the cathodic reduction of SO.sub.2 on an inert cathode the following process occurs, as expressed by the general equation: EQU SO.sub.2 +2e+2Li.fwdarw.Li.sub.2 S.sub.2 O.sub.4 .dwnarw.
The resulting lithium dithionite is sparingly soluble in the electrolyte and deposited on the surface of the inert electrode. The deposition of lithium dithionite results in blocking and passivation of the electrode surface, and, consequently, to deceleration of the process of cathodic reduction of sulphur dioxide (cf. U.S. Pat. No. 3,953,225).
As it has been demonstrated by the results of cyclic voltammetry on a smooth electrode, the more negative the potential of change of the scanning direction from the cathodic polarization to the anodic one, the more the anodic peak of oxidation of the products of SO.sub.2 reduction becomes shifted towards the positive area and the higher is the value of this peak.
Similar relationships are inherent in the electrochemical reactions, wherein the products are not dissolved in the electrolyte, but accumulated on the electrode.
A similar process also occurs in a current source with another active substance dissolved in an electrolyte, in a source of the system Li-SOCl.sub.2, wherein the active substance of the cathode is itself a solvent.
The paper by A. N. Dey and P. Bro ("Primary Li/SOCl.sub.2 cells" J. Electrochem. Soc., vol. 125, No. 10) indicates that during the cathodic reduction of SOCl.sub.2 on an inert cathode the following general reaction proceeds: EQU 4Li.sup.+ +2SOCl.sub.2 .fwdarw.4LiCl.dwnarw.+SO.sub.2 +S.dwnarw.
The solid phases of LiCl and S deposited on the cathode surface provide a passivating effect thereupon.
In order to optimize performances of a current source, wherein during the cathodic reaction solid-phase products are formed, as the inert electrodes use is made of porous electrodes having a developed inner surface.
The solid-phase reaction product is distributed inside the pores and the process of the surface passivation is manifested to a considerably lesser extent than on a smooth electrode.
In the above-discussed papers devoted to studying the cathodic reduction of SO.sub.2 and SOCl.sub.2 it has been pointed out that the deposition of the reaction product inside the electrode pores results in swelling and has changed thickness and elasticity as well.
At the same time, due to propagation of the process inside the electrode, the true current density at the electrode surface is smaller than the calculated one. This results in the fact that the porous electrode within a specific range of current density values depending on the electrochemical properties of a given system and the electrode structure has a lowered polarization value as compared to a smooth electrode.
In current sources, wherein the product of a cathodic reaction is deposited on the cathode surface as a solid phase it is the cathodic process that limits their specific characteristics. In this connection, the microstructure of a porous inert electrode, while influencing the cathodic process course, also defines the efficiency of operation of the chemical source of current on the whole.
On a porous electrode the electrochemical process is distributed non-uniformly with respect to the electrode thickness. The process speed is maximum at the front surface and gradually lowered along the electrode depth due to the effect of the electrolytic resistance inside the pores and diffusion limitations relative to the active substance in pores.
With elevation of the current density the process nonuniformity is enhanced and it is expelled towards the front surface. The apparent working surface is reduced, polarization is increased and all the limiting factors are pronounced to a greater extent. In the case of the formation of a solid-phase reaction product the process is complicated by clogging of the pores with insoluble matter which hingers the propagation of the process inside the electrode.
Therefore, in contrast to a smooth electrode, the superficial layer of a porous electrode takes part in the electrochemical process and its thickness depends on a whole number of factors including structure of the electrode, its properties and the current density.
As has been already mentioned hereinabove, in the formation of a solid-phase product of the reaction of cathodic reduction of, e.g. SO.sub.2 to lithium dithionite Li.sub.2 S.sub.2 O.sub.4, the cathodic process limits the characteristics of the current source of the Li-SO.sub.2 system. By changing the characteristics of the inert cathode whereupon the reduction of the dissolved active substance takes place, it is possible to considerably increase the power and energy capacity of the current source.
At present, inert porous electrodes are manufactured from carbonaceous or metal materials (cf. U.S. Pat. No. 3,892,589). However, electrodes from carbonaceous materials have found more extensive application. They are produced by any of the following processes: compression-molding, spraying, spreading, suction-on out of pulp. In all these processes a carbonaceous material is preliminarily disintegrated to a required particle size, mixed with a binder and, when necessary, with a pore-forming agent and/or hydropho- agent: in some processes a further heat treatment is effected. Thus, USSR Inventor's Certificate No. 459820 teaches a process for the manufacture of an inert electrode of a primary cell involving application onto a metal substrate of a suspension of a mixture of carbon black and graphite with a solution of a polymer in an organic solvent, followed by drying the thus applied layer.
Known in the art is an electrochemical cell (cf. U.S. Pat. No. 3,891,458, wherein the anode is made of Zn (Li, Mg, Na, Ca, Al) and the electrolyte and active cathodic substance is SOCl.sub.2. The cathodic reduction of the active substance present in the liquid phase occurs on the surface of an inert cathode consisting of a three-dimensional Ni grate with a layer of a mixture of 80% by mass of acetylene carbon black, 17% by mass of graphite and 3% by mass of a binder deposited onto the grate by means of hot compression molding at the temperature of 200.degree. C.
However, these electrochemical cells do not possess the required discharge capacity and power.
Increased values of specific power and energy capacity can be achieved through minimization of the passivation of the surface of the inert electrode with the solidphase reaction product by means of variation of the electrode macrostructure, increasing the concentration of the active substance dissolved in the electrolyte, and the formation of a soluble cathodic product non-passivating the electrode surface.
U.S. Pat. No. 3,929,507 teaches a Li-SO.sub.2 current source, wherein the electrolyte is made of a solution of lithium bromide in a mixture of acetonitrile and propylene carbonate.
Known in the art is a paper (Bro P., Kang H.Y., Schlaikjer C., Taylor H., High rate Li/SO.sub.2 batteries "Record 10.sup.th Intersec. Energy Convers. Eng. Conf., Newark Del., 1975," New York, 1975, 432-436), wherein for a Li-SO.sub.2 cell an electrolyte is used with various proportions of propylene carbonate, acetonitrile and lithium bromide from the standpoint of an optimal electrically conducting system.
An electrochemical cell is known, comprising sulphur dioxide as a cathodic depolarizer (cf. U.S. Pat. No. 3,567,515).
The anode is made of an alkali metal and the cathode from a carbonaceous material with a developed surface.
Depending on the nature of the salt which is dissolved in an organic solvent, the products of cathodic reduction can be either soluble or insoluble.
Insoluble products are formed when using salts of alkali metals and tetraalkylammonium. In case insoluble products are formed, porous inert cathodes are used.
As the organic solvents use is made of propylene carbonate and acetonitrile, mixtures thereof and the like.
However, the electrolytes employed in this cell provide a relatively low solubility of sulphur dioxide.
At a low solubility of sulphur dioxide in the electrolyte of an electrochemical cell, pressure tension sulphur dioxide vapours over the solution is sharply increased with a rise in temperature and, hence, the pressure in the system increases accordingly. This, on the one hand, limits the permissible working temperature range and, on the other hand, necessitates an increase in the mechanical strength of the cell housing, thus resulting in an increased weight of the current source.
Furthermore, this electrochemical cell has insufficient discharge characteristics, specific power capacity and power.